1. Field of the Invention
The present invention relates to a method of and apparatus for quantitative analysis in accordance with CARS (Coherent Anti-Stokes Raman Spectroscopy).
2. Description of the Prior Art
CARS is a kind of non-linear Raman spectroscopy which has progressed with the development of highpower laser, and dye laser, and which exhibits a detection sensitivity of around 10.sup.5 times greater than that of conventional Raman spectroscopy so that CARS is expected to be applied in various fields.
The principle of CARS will be simply described hereinbelow. As illustrated in FIG. 1, when laser light for excitation having frequency .omega..sub.1 is irradiated on a material (Raman-active material) RM together, with laser light of the same frequency (Stokes' frequency) .omega..sub.2 (=.omega..sub.1 -.OMEGA., wherein .OMEGA. is the proper frequency of a molecule of the material RM) as that of Stokes' light of the material RM, very intensive, beam-like anti-Stokes' light (frequency .omega..sub.3 =.omega..sub.1 +.OMEGA.) of the material RM is resonantly generated. Such a phenomenon may be interpreted as a four-photon process as illustrated in the energy diagram of FIG. 2. Furthermore, an intensity I.sub.3 of the anti-Stokes' light at that time is given by the following expression: EQU I.sub.3 .varies.I.sub.1.sup.2 .multidot.I.sub.2 .multidot.N.sup.2 ( 1)
wherein N is the density of the material RM, I.sub.1 is the intensity of the laser light having frequency .omega..sub.1 and I.sub.2 is the intensity of the laser light having frequency .omega..sub.2. Accordingly, when the intensity I.sub.3 of the aforesaid anti-Stokes' light is detected, the concentration of the material RM can be detected. CARS is based on the above-described principle whereby the concentration of a material is measured by detecting the intensity I.sub.3 of the aforesaid anti-Stokes' light.
Light detected in accordance with CARS varies remarkably, in actuality, dependent upon variation in intensity of the laser light for excitation, conditions of the optical system and the like. Accordingly, an apparatus for quantitative analysis (apparatus for measuring concentration) in accordance with conventional CARS is constructed in such a manner that signal strength from a system to be measured is corrected by utilizing the signal from a reference cell containing a known reference material as the standard, whereby concentration is measured.
FIG. 3 is a schematic view showing an apparatus for quantitative analysis in accordance with conventional CARS wherein a cell A is filled with a material to be measured, while a cell B is filled with a reference material of a prescribed concentration. The laser light for excitation of frequency .omega..sub.1 is reflected with a mirror 1 to be introduced to a beam splitter 2', whilst laser light of frequency .omega..sub.2 is applied to a dichroic mirror 2. The laser light for excitation of frequency .omega..sub.1 and the laser light of .omega..sub.2 are synthesized by means of the dichroic mirror 2, then the resulting synthesized light is split by means of a beam splitter 2' into two portions, a portion of which is applied to a spectrograph 7 through a lens 4, the cell A, a lens 5 and a prism 6, whilst the other portion is applied to a spectrograph 11 through a lens 8, the cell B, a lens 9 and a prism 10 after reflection off of mirror 3. The spectrographs 7 and 11 are provided with detectors 7a and 11a, respectively, and these detectors detect the light intensity being applied to the spectrographs 7 and 11 and convert this intensity into electrical signals.
In the conventional system as mentioned above, however, two systems for signal detection consisting of a first signal detection system involving the spectrograph 7 and the detector 7a and a second signal detection system involving the spectrograph 11 and the detector 11a are required. Also, adjustment of the optical axis must also be effected by means of two systems. Therefore, such a conventional system becomes expensive and adjustment requires much labor.
Furthermore, in the above-described measurement, it is also required that the Raman wavenumber (corresponding to the Stokes' frequency) of a reference material be in the vicinity (within a range of 80 cm.sup.-1) of that of a material to be determined in order to determine the signal strength of the material to be determined at the same time of that of the reference material.
Supposing that, for example, concentration of hydrogen is measured in accordance with CARS, since the hydrogen molecule has a very high vibrational level of 4169 cm.sup.-1 as compared with those of other materials, there is no suitable reference material satisfying the above-described condition so that such measurement in which any reference material is utilized could not have heretofore been effected. Notwithstanding this fact, if there is no correction on the basis of a reference material, the measured values obtained involve variation in the intensity of laser light for excitation, or in the conditions of the optical system so that such measurement is not reliable.